What are Sea Urchins?

About Sea Urchins:

• Sea urchins belong to a group of marine invertebrates called echinoderms, which means spiny-skinned animals. 
• It includes other well-known marine creatures like starfish and sea cucumbers.
• Sea urchins are characterized by their spherical to somewhat flattened, spiny bodies, and they are found in oceans worldwide, from shallow coastal waters to deep-sea environments.
• They live on the ocean floor, usually on hard surfaces, and use tube feet or spines to move about.
• The largest urchin (known from a single specimen) is Sperostoma giganteum of deep waters off Japan.
• Features:
  • They have a globular body and a radial arrangement of organs, shown by five bands of pores running from mouth to anus over the test (internal skeleton).
  • The pores accommodate tube feet, which are slender, extensible, and often sucker-tipped. 
  • They have a hard exoskeleton, or test, made up of interlocking plates or ossicles, which are often covered with movable spines.
  • From nodules on the test arise long, movable spines and pedicellariae (pincerlike organs); these structures may have poison glands. 
  • Feeding:
    • They are herbivorous, primarily feeding on algae and plant material.
• They use their specialized mouthparts, called Aristotle's lantern, to scrape algae and other food sources from rocks or the seafloor.

Q1) What is algae? 

Algae are a diverse group of photosynthetic microorganisms and sometimes macroscopic organisms that primarily live in aquatic environments, although they can also be found in terrestrial habitats. They are not considered plants, but they share some similarities with plants, particularly in their ability to perform photosynthesis.

Source: Red Sea corals threatened by mystery sea urchin deaths